Recently, research into improving capacity of lithium secondary batteries has been actively conducted as lithium secondary batteries have been used as power sources of vehicles as well as portable electronic devices such as mobile phones, personal digital assistants (PDAs), and laptop computers. In particular, demands for increasing capacity of lithium secondary batteries have been further increased as an amount of energy consumption has been increased according to multi-function of portable electronic devices, and development of a high-capacity lithium secondary battery able to stably maintain power in a state of charge (SOC) range along with high power is continuously required in order to be used as a power source of a medium and large sized device, such as a hybrid electric vehicle (HEV), a plug-in hybrid electric vehicle (PHEV), and an electric vehicle (EV).
A battery using lithium cobalt-based oxide as a cathode active material among these lithium secondary batteries is the most widely used due to excellent electrode lifetime and high high-rate charge and discharge efficiency. However, since high-temperature safety of the lithium cobalt oxide is low and cobalt used as a raw material is a relatively expensive material, there may be a limitation in price competitiveness.
Accordingly, lithium-containing manganese oxide has been proposed as a cathode active material. In particular, spinel-structured lithium-containing manganese oxide has advantages in that thermal stability may be excellent, the price may be low, and the synthesis thereof may be facilitated. However, the spinel-structured lithium-containing manganese oxide has disadvantages in that capacity may be low, lifetime characteristics may be degraded due to a side reaction, and cycle characteristics and high-temperature characteristics may be poor.
As a result, layer-structured lithium-containing manganese oxide is suggested in order to compensate for the low capacity of the spinel and secure excellent thermal safety of manganese-based active materials. In particular, layer-structured aLi2MnO3-(1−a)LiMO2 having a content of manganese (Mn) greater than those of other transition metal(s) may have relatively high initial irreversible capacity. However, relatively high capacity may be manifested during charging at a voltage of 4.4 V or more based on a cathode potential.
That is, in the case that overcharging is performed at a high voltage of 4.4 V or more (for example, 4.5 V or more) based on a cathode potential during initial charging, the layer-structured lithium-containing manganese oxide exhibits a high capacity of over 250 mAh/g as well as an excessive amount of gas, such as oxygen and carbon dioxide, being generated, while exhibiting a plateau potential range of 4.5 V to 4.8 V.
Thus, some of remaining lithium (Li) and transition metals may migrate due to the excessive deintercalation of lithium ions and release of oxygen during the charging at a high voltage of 4.4 V or more based on a cathode potential, and a phase transition into a spinel-like structure may be inferred through this process. In particular, it is estimated that additional manifestation of capacity near 3V in this case may be due to the transition into a spinel-like structure.
However, in the case that the transition into a spinel-like structure occurs, since electrical conductivity is insufficient in 3V region (2.7 V to 3.1 V), desired output characteristics and cycle durability may not be secured by itself.
Therefore, with respect to a cathode active material including the layer-structured lithium-containing manganese oxide, there is an urgent need to develop a cathode active material able to improve output characteristics and cycle durability in the 3V region by preventing a decrease in electrical conductivity due to the structural change after a first charge and discharge cycle.